Interior permanent magnet electric machine

ABSTRACT

An interior permanent magnet electric machine is described, and includes a stator including a plurality of electrical windings and a rotor disposed in a cylindrically-shaped void formed within the stator. The rotor includes a plurality of steel laminations assembled onto a shaft, wherein the shaft defines a longitudinal axis. Each of the steel laminations includes a plurality of poles and each of the poles includes a plurality of slots disposed near an outer periphery. The slots of the steel laminations are longitudinally aligned. A plurality of permanent magnets are disposed in a first subset of the slots, and plurality of packets assembled from anisotropic material are disposed in a second subset of the slots.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/291,876 filed on Feb. 5, 2016, the disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to interior permanent magnet electric machines, and more specifically to a rotor assembly for such electric machines.

BACKGROUND

An interior permanent magnet machine generally includes a rotor that is rotatably disposed within a stator, wherein the rotor has a plurality of magnets of alternating polarity placed around an outer periphery of the rotor. The stator generally includes a plurality of windings that produces electromagnetic poles of alternating polarity when excited with electrical current. Permanent magnet electric machines may have a relatively high torque/power density and other properties that cause them to be preferred over other devices such as wound-field synchronous machines with brushed or brushless exciters. However, permanent magnet electric machines may employ high volumes of magnetic material.

SUMMARY

An interior permanent magnet electric machine is described, and includes a stator including a plurality of electrical windings and a rotor disposed in a cylindrically-shaped void formed within the stator. The rotor includes a plurality of steel laminations assembled onto a shaft, wherein the shaft defines a longitudinal axis. Each of the steel laminations includes a plurality of poles and each of the poles includes a plurality of slots disposed near an outer periphery. The slots of the steel laminations are longitudinally aligned. A plurality of permanent magnets are disposed in a first subset of the slots, and plurality of packets assembled from anisotropic material are disposed in a second subset of the slots.

The above features and advantages, and other features and advantages, of the present teachings are readily apparent from the following detailed description of some of the best modes and other embodiments for carrying out the present teachings, as defined in the appended claims, when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates a cross-sectional view of a permanent magnet electric machine including an annular-shaped stator that forms a cylindrically-shaped void into which a coaxial rotor is inserted, in accordance with the disclosure;

FIG. 2 schematically illustrates a cross-sectional view of a single pole portion of a single lamination of the permanent magnet electric machine described with reference to FIG. 1 including a pole arrangement including a plurality of slots that are disposed near an outer periphery thereof, wherein permanent magnets or anisotropic packets are inserted into subsets of the slots, in accordance with the disclosure;

FIG. 3 schematically shows a perspective view of a first embodiment of the anisotropic packet, which includes a lamination stack composed of a plurality of anisotropic laminations having interspersed non-magnetic spacers, in accordance with the disclosure;

FIG. 4 schematically shows a perspective view of a second embodiment of the anisotropic packet, which includes a lamination stack composed of a plurality of anisotropic laminations, each having a plurality of voids that function as flux barriers, in accordance with the disclosure; and

FIG. 5 schematically shows a perspective view of a third embodiment of the anisotropic packet, which includes a lamination stack composed of a plurality of anisotropic laminations, in accordance with the disclosure.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the depictions are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same, FIG. 1 schematically illustrates a cross-sectional view of a permanent magnet electric machine 10 including an annular-shaped stator 12 that forms a cylindrically-shaped void into which a coaxial rotor 20 is inserted, with an airgap 14 formed between the stator 12 and the rotor 20. The rotor 20 includes a rotatable shaft 26 having ends that extend through apertures formed in endcaps of the stator 12, with suitable mounting and bearing devices arranged therein. The shaft 26 defines a longitudinal axis with radial lines extending orthogonal to the longitudinal axis. The stator 12 houses a plurality of electrical windings (not shown) that are circumferentially arranged. The electrical windings are electrically connected to a suitable device, e.g., an inverter, which may be controlled by a controller to generate rotating electrical fields that induce magnetic fields adjacent to the rotor 20 and interact with the magnetic field of the permanent magnets to generate mechanical torque in the rotor 20. The electric machine 10 may be controlled to operate as a torque motor and/or an electric power generator. Details related to a physical configuration and operation of a permanent magnet electric machine 10 are known to those skilled in the art, and thus not described in detail herein.

The rotor 20 is composed of a plurality of disc-shaped laminations 22 that are fabricated from isotropic steel or another ferromagnetic material such as compressed iron powder. The plurality of laminations are stacked and press-fit or otherwise fixedly assembled onto the shaft 26 to rotate in concert therewith. The rotor 20 includes a plurality of pole portions 25 that are circumferentially located about the rotor shaft 26, preferably at predetermined spacings that are identical. One pole portion 25 of a single lamination 22 is indicated with reference to FIG. 1 and is schematically shown in additional detail with reference to FIG. 2. Embodiments of the rotor 20 may have two pole portions 25, four pole portions 25, six pole portions 25, eight pole portions 25, or any other suitable quantity of pole portions 25.

The pole portion 25 of the single lamination 22 of the rotor 20 includes a pole arrangement 30 including a plurality of slots 31 arranged in layers 31 a, 31 b, 31 c and 31 d as shown that are disposed near an outer periphery 24 thereof, wherein the layers 31 a, 31 b, 31 c and 31 d are defined in relation to the outer periphery 24. Four layers are shown, but any quantity of layers may be employed. When the plurality of laminations are assembled onto the shaft 26, the slots 31 are aligned and are arranged parallel to the longitudinal axis defined by the shaft 26. Devices may be inserted into some or all of the slots 31, and a subset of the plurality of slots 31 may be unfilled and thus may function as flux barriers.

When the rotor 20 is employed on the permanent magnet electric machine 10, permanent magnets 34 may be inserted into a subset of the slots 31, e.g., the slots in layers 31 c and 31 d as shown, and thus define the poles of each of the pole arrangements 30. Each of the pole arrangements 30 defines a direct or d-axis 28 and a quadrature or q-axis 29, wherein the d-axis 28 is aligned with the center of the magnetic pole and the q-axis 29 is orthogonal to the d-axis 28 and aligned with a mid-point of two magnetic poles of the rotor. The d-axis 28 indicates an orientation having the lowest inductance, and the q-axis 29 indicates an orientation having the highest inductance. As such, there is a d-axis 28 and a q-axis 29 associated with each of the pole arrangements 30.

A saliency ratio is defined as follows: ξ,=L _(q) /L _(d)

wherein

-   -   ξ, is the saliency ratio,     -   L_(q) is the inductance along the d-axis 28, and     -   L_(d) is the inductance along the q-axis 29.

As known to those skilled in the art, performance of a permanent magnet electric machine improves with increased saliency ratio.

Anisotropic inserts 36 may be inserted into a second subset of the slots 31, e.g., the slots in layers 31 a and 31 b as shown. A face portion 38 of one of the anisotropic packets 36 and a lamination rolling direction 40 are indicated.

A subset of the slots 31 may remain void in one embodiment. Alternatively, all of the slots 31 may contain either a permanent magnet 34 or an anisotropic packet 36, with none of the slots 31 remaining void.

FIG. 3 schematically shows a first embodiment of the anisotropic packet 36, which includes a lamination stack 42 composed of a plurality of anisotropic laminations 44 having interspersed non-magnetic spacers 46, wherein the anisotropic laminations 44 are layered axially. A face portion 38 of the anisotropic packet 36 is identified, and indicates that portion of the anisotropic packet 36 that is visible on either end of the rotor 20. An anisotropic material is a material that exhibits unequal physical properties along different axes. The anisotropic laminations 44 described herein preferably exhibit magnetic properties related to induction permeability that change depending upon the rolling direction of the material when it is processed. Specifically, the anisotropic laminations 44 described herein preferably exhibit increased induction permeability, increased magnetization and increased flux density in the rolling direction as compared to orthogonal directions of the material. The plurality of anisotropic laminations 44 may be fabricated from a grain-oriented steel material in one embodiment. In one embodiment, the grain-oriented steel material includes an iron-silicon magnetic alloy that is processed by cold rolling, which serves to achieve the preferred magnetic properties related to induction permeability, magnetization and increased flux density. Alternatively, the plurality of anisotropic laminations 44 may be fabricated from an amorphous metal alloy that is formed employing a rapid solidification process.

The lamination rolling direction 40 is indicated for the plurality of anisotropic laminations 44, as are the preferred predominant d-axis 28 and q-axis 29 of the rotor 20 when the anisotropic packet 36 is inserted into the rotor 20. The non-magnetic spacers 46 may be fabricated by oxide deposition onto surfaces of the anisotropic laminations 44, or may be fabricated from aluminum or alumina, or include both, depending on thickness required. The thicknesses of the anisotropic packets 36 and the non-magnetic spacers 46 are preferably determined using motor simulation under simulated motor load conditions, taking into account magnetic properties, permeability, temperature, torque generation, and other factors. A depth 50 of the anisotropic packet 36 is also shown, and indicates the length that the anisotropic packet 36 projects into the slot 31 of the rotor 20. Preferably, and as shown the axis of low magnetic reluctance of the anisotropic packet 36 is aligned with the q-axis 29 of the rotor 20, which corresponds to the cold rolling direction 40 for the plurality of anisotropic laminations 44. The depth 50 of the anisotropic packet 36 may be limited in order to reduce eddy current loss and facilitate insertion.

Alternatively, the first embodiment of the anisotropic packet 36 may include the lamination stack 42 composed of a plurality of anisotropic laminations 44 wherein the anisotropic laminations 44 are layered axially without interspersed non-magnetic spacers.

FIG. 4 schematically shows a second embodiment of the anisotropic packet 136, which includes a lamination stack 142 composed of a plurality of anisotropic laminations 144, each having a plurality of voids 146 that function as flux barriers wherein the anisotropic laminations 144 are layered radially. A face portion 138 of the anisotropic packet 136 is identified, and indicates that portion of the anisotropic packet 136 that is visible on either end of the rotor 20. The voids 146 may be fabricated using any suitable technology, e.g., stamping or laser cutting. Each of the anisotropic laminations 144 has a planar surface into which the voids 146 are fabricated. The planar surfaces of the stacked anisotropic laminations 144 have shapes that are analogous to a cross-sectional shape of at least a portion of the associated slot 31 of the rotor 20 into which the anisotropic packet 136 is assembled.

Each of the plurality of anisotropic laminations 144 is preferably fabricated from a grain-oriented steel material. In one embodiment, the grain-oriented steel material includes an iron-silicon magnetic alloy that is processed by cold rolling, which serves to achieve preferred magnetic properties related to permeability in the rolling direction. Alternatively, the plurality of anisotropic laminations 144 may be fabricated from an amorphous metal alloy that is formed employing a rapid solidification process.

The lamination rolling direction 40 is indicated for the plurality of anisotropic laminations 144, as are the preferred predominant d-axis 28 and q-axis 29 of the rotor 20 when the anisotropic packet 136 is inserted into the rotor 20. The thickness of the anisotropic packets 136 is preferably determined using motor simulation under simulated motor load conditions, taking into account magnetic properties, permeability, temperature, torque generation, and other factors. A depth 50 of the anisotropic packet 136 is also shown, and indicates the length that the anisotropic packet 136 projects into the slot 31 of the rotor 20. Preferably, and as shown the q-axis 29 of the rotor 20 is aligned with the axis of low magnetic reluctance of the anisotropic packet 136, which corresponds to the cold rolling direction 40 for the plurality of anisotropic laminations 144. As such, this embodiment of the anisotropic packet 136 includes a lamination stack 142 composed of a plurality of anisotropic laminations 144, each having a plurality of voids 146 that function as flux barriers, with a q-axis 29 of the rotor 20 aligned with the axis of low magnetic reluctance, which corresponds to the cold rolling direction for the material. No spacer is required between the individual anisotropic laminations 144. However, the end portion of the lamination stack 142 presents no barriers in the path of the d-axis flux. This leakage path is in the direction of low material permeance, perpendicular on the rolling direction, so leakage will be relatively low.

FIG. 5 schematically shows a third embodiment of the anisotropic packet 236, which includes a lamination stack 242 composed of a plurality of anisotropic laminations 244. In one embodiment, a plurality of non-magnetic spacers 246 may be interspersed with the anisotropic laminations 244. Alternatively, no non-magnetic spacers 246 are employed. A face portion 238 of the anisotropic packet 236 is identified, and indicates that portion of the anisotropic packet 236 that is visible on either end of the rotor 20. Each of the anisotropic laminations 244 has a planar surface having a shape that is analogous to a cross-sectional shape of at least a portion of the associated slot 31 of the rotor 20 into which the anisotropic packet 236 is assembled. Each of the plurality of anisotropic laminations 244 is preferably fabricated from a grain-oriented steel material. In one embodiment, the grain-oriented steel material includes an iron-silicon magnetic alloy that is processed by cold rolling, which serves to achieve preferred magnetic properties related to permeability in the rolling direction. Alternatively, the plurality of anisotropic laminations 244 may be fabricated from an amorphous metal alloy that is formed employing a rapid solidification process.

The lamination rolling direction 40 is indicated for the plurality of anisotropic laminations 244, as are the preferred predominant d-axis 28 and q-axis 29 of the rotor 20 when the anisotropic packet 236 is inserted into the rotor 20. A depth 50 of the anisotropic packet 236 is also shown, and indicates the length that the anisotropic packet 236 projects into the slot 31 of the rotor 20. Preferably, and as shown the q-axis 29 of the rotor 20 is aligned with the axis of low magnetic reluctance of the anisotropic packet 236, which corresponds to the cold rolling direction 40 for the plurality of anisotropic laminations 244.

This disclosure improves rotor saliency of an interior permanent magnet rotor, which results in reduced dependency on the permanent magnet, thus permitting a reduction in mass of the magnets to achieve similar performance in some embodiments. Alternatively, electric machine performance in form of motor torque-per-ampere and efficiency may be improved. Furthermore, there may be improved saliency of an embodiment of an interior permanent magnet electric machine due to the use of grain-oriented steel laminations that are axially inserted in the rotor slots. The grain-oriented steel laminations are placed axially in the rotor slots of radial lamination resulting in a hybrid structure with axial and radial laminations. A high saliency ratio (L_(q)>>L_(d)) may increase torque production and improve electric machine efficiency. Increased saliency also improves high speed performance of the electric machine. The increase in rotor saliency is achieved by increasing the q-axis inductance by the axial insertion of grain oriented lamination, while keeping the d-axis inductance unaffected. The use of the radial lamination and axial lamination packets facilitate higher pole count increased torque per ampere and increased peak motor torque or a reduction of motor size and magnet mass to achieve similar performance.

As used in this specification and claims, the terms “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation. Furthermore, in reading the claims it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim.

It should be understood that while the use of words such as preferable, preferably, preferred or more preferred utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the disclosure, the scope being defined by the claims that follow.

The detailed description and the drawings or figures are supportive and descriptive of the present teachings, but the scope of the present teachings is defined solely by the claims. While some of the best modes and other embodiments for carrying out the present teachings have been described in detail, various alternative designs and embodiments exist for practicing the present teachings defined in the appended claims. 

The invention claimed is:
 1. An interior permanent magnet electric machine, comprising: a stator including a plurality of electrical windings; a rotor disposed in a cylindrically-shaped void formed within the stator and including a plurality of steel laminations assembled onto a shaft, wherein the shaft defines a longitudinal axis, wherein each of the steel laminations includes a plurality of poles and wherein each of the poles includes a plurality of slots disposed near an outer periphery, wherein the slots of the steel laminations are longitudinally aligned; a plurality of permanent magnets; and a plurality of packets assembled from anisotropic material; wherein the plurality of permanent magnets are disposed in a first subset of the slots; and wherein the plurality of packets assembled from anisotropic material are disposed in a second subset of the slots.
 2. The interior permanent magnet electric machine of claim 1, wherein the plurality of packets assembled from anisotropic material are disposed in the second subset of the slots in an axial orientation that is disposed parallel to the longitudinal axis.
 3. The interior permanent magnet electric machine of claim 1, wherein each of the packets assembled from anisotropic material comprises a stack including a plurality of laminations fabricated from a grain-oriented steel material, wherein the grain-oriented steel material of the plurality of laminations are aligned along a common axis.
 4. The interior permanent magnet electric machine of claim 3, wherein the common axis associated with the grain-oriented steel materials of the plurality of laminations is parallel with a d-axis associated with one of the poles of the rotor.
 5. The interior permanent magnet electric machine of claim 3, wherein the common axis associated with the grain-oriented steel materials of the plurality of laminations is aligned with an axis of low magnetic reluctance associated with one of the poles of the rotor.
 6. The interior permanent magnet electric machine of claim 3, wherein the plurality of laminations fabricated from a grain-oriented steel material are layered radially.
 7. The interior permanent magnet electric machine of claim 3, wherein the plurality of laminations fabricated from a grain-oriented steel material are layered axially.
 8. The interior permanent magnet electric machine of claim 3, wherein the grain-oriented steel material comprises an iron-silicon magnetic alloy.
 9. The interior permanent magnet electric machine of claim 1, wherein a third subset of the slots remain void.
 10. The interior permanent magnet electric machine of claim 1, wherein each of the steel laminations comprises a disc-shaped steel lamination.
 11. The interior permanent magnet electric machine of claim 10, wherein each of the disc-shaped steel laminations are fabricated from an isotropic material.
 12. A rotor for an interior permanent magnet electric machine, the rotor comprising: a plurality of steel laminations assembled onto a shaft, wherein the shaft defines a longitudinal axis, wherein each of the steel laminations includes a plurality of poles, wherein each of the poles includes a plurality of slots disposed near an outer periphery, and wherein the slots of the steel laminations are longitudinally aligned; a plurality of permanent magnets; and a plurality of packets assembled from anisotropic material, wherein the plurality of permanent magnets are disposed in a first subset of the slots and wherein the plurality of packets assembled from anisotropic material are disposed in a second subset of the slots.
 13. The rotor of claim 12, wherein the plurality of packets assembled from anisotropic material are disposed in the second subset of the slots in an axial orientation that is disposed parallel to the longitudinal axis.
 14. The rotor of claim 12, wherein each of the packets assembled from anisotropic material comprises a stack including a plurality of laminations fabricated from a grain-oriented steel material, wherein the grain-oriented steel material of the plurality of laminations are aligned along a common axis.
 15. The rotor of claim 14, wherein the common axis associated with the grain-oriented steel materials of the plurality of laminations is parallel with a d-axis associated with one of the poles of the rotor.
 16. The rotor of claim 14, wherein the common axis associated with the grain-oriented steel materials of the plurality of laminations is aligned with an axis of low magnetic reluctance associated with one of the poles of the rotor.
 17. The rotor of claim 14, wherein the plurality of laminations fabricated from a grain-oriented steel material are layered radially.
 18. The rotor of claim 14, wherein the plurality of laminations fabricated from a grain-oriented steel material are layered axially.
 19. The rotor of claim 12, wherein a third subset of the slots remain void.
 20. The rotor of claim 12, wherein the grain-oriented steel material comprises an iron-silicon magnetic alloy. 